1. Field of Invention
This invention relates to laser detection. Specifically, the present invention relates to laser warning systems used to distinguish threatening laser energy from other laser energy.
2. Description of the Related Art
Laser detection systems are used in a variety of demanding applications ranging from tactical military applications to police laser guns used to measure automobile speed. Such systems require reliable sensors and associated electronics that can consistently distinguish between important signals and stray or reflected signals.
A laser is a device that that emits a beam of electromagnetic energy having certain special properties. Laser energy is light that is collimated, coherent, and monochromatic, i.e., is comprised of waves having similar frequencies and phases. In the atmosphere, air molecules may scatter laser energy. The scattered, often incoherent laser energy is often detectable over one hundred feet away from the original laser beam. A typical laser detector in a laser warning system can detect the main laser beam in addition to energy scattered therefrom.
Ground combat applications require systems that can readily distinguish between direct and reflected laser energy. To detect laser energy, a Fabry Perot etalon is often employed to determine the coherence of laser energy. A Fabry-Perot etalon is disclosed in U.S. Pat. No. 4,309,108 entitled ANALYZER FOR COHERENT RADIATION, issued Jan. 5, 1982 to Edward T. Siebert, the teachings of which are herein incorporated by reference. A laser detection system employing a Fabry-Perot etalon uses coherence and intensity to separate threatening laser light from stray or reflected electromagnetic energy. A skimming beam having an indirect angle of arrival may not provide sufficient contrast to trigger a detection. Also, dim light having an intensity below a given threshold is rejected. However, laser energy reflected from objects on the ground and received by a laser warning system on a tank, for example, may have sufficient intensity to trigger a detection or alarm. If the laser energy reflects from an adjacent friendly tank, the laser warning system may initiate counter-measures with respect to the friendly tank. This is distracting and sometimes dangerous. Hence, systems relying exclusively on the Fabry-Perot etalon may fail to consistently distinguish direct laser light from reflected laser light.
Hence, a need exists in the art for a system that can readily distinguish between an actual laser threat and stray or reflected electromagnetic energy. There is a further need for a laser warning system that is adapted for use in ground based applications.